Effect of fibre concentration, strain rate and weldline on mechanical properties of injection-moulded short glass fibre reinforced thermoplastic polyurethane

The effect of fibre concentration, strain rate and weldline on tensile strength, tensile modulus and fracture toughness of injection-moulded thermoplastic polyurethane (TPU) reinforced with different concentration levels of short glass fibres was investigated. It was found that tensile strength, σc, of single-gated mouldings increased with increasing volume fraction of fibres, ϕf, according to a second order polynomial function of the form \( \sigma_{\text{c}} = a₀ + a₁ \phi_{\text{f}} + a₂ \phi_{\text{f}}² \) and increased linearly with natural logarithm of strain rate (\( { \ln }\,\dot{e} \)). Tensile modulus and fracture toughness (at initiation) of single-gated mouldings increased linearly with increasing ϕf (rule-of-mixtures) and \( { \ln }\,\dot{e} \). A linear dependence was obtained between fibre efficiency parameter for composite modulus, ηE, and \( { \ln }\,\dot{e} \). The presence of weldline in double-gated mouldings reduced tensile strength, tensile modulus and fracture toughness of TPU composites but had no significant effect upon properties of the TPU matrix. All the aforementioned properties increased with increasing fibre concentration and showed a linear dependence with respect to \( { \ln }\,\dot{e} \). Weldline integrity factor for all three properties decreased with increasing fibre concentration showing no strain-rate effect of any significance. Results indicated that tensile strength was more affected by the presence of weldline than tensile modulus or fracture toughness. It was noted that composite properties in the presence of weldline were still much greater than those for the unweld matrix. Weldline integrity values close to unity indicated that measured properties for the matrix were not significantly affected by the weldline.